Direct Electrochemical Sensing of Lipopolysaccharide by Graphene Nanoplatlets@Cytochrome c Hydrogel Using a Microfluidic Chip

Contamination by lipopolysaccharide (LPS), an endotoxin which is present in the outer membrane of Gram-negative bacteria causes adverse toxic effects on human health. Herein, we report a facile graphene nanoplatelets@cytochrome c (GNP@Cyt c) hydrogel based voltammetric electrochemical sensor for LPS. Firstly, the resultant hydrogel sensor facilitated a large electroactive surface coverage for electron transfer, and secondly provided a 3D porous microenvironment for interaction of abundant electrocatalytic sites in Cyt c with LPS. Under optimal incubation conditions, the heme iron of Cyt c in its reduced form (Fe2+) can directly bind to highly anionic carbohydrate groups of LPS, thereby giving rise to specific interaction. Moreover, the binding of Cyt c (Fe2+) with LPS served as a electrocatalytic unit, generating a strong oxidation peak current at 0.019 V. Importantly, no separate redox mediator was required and the electrochemical signal obtained by LPS-Cyt c (Fe2+) complex sufficed for detection purposes. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were employed to investigate the electrochemical aspects of GNP@Cyt c hydrogel modified screen-printed carbon electrode (SPCE). Further, the GNP@Cyt c hydrogel/SPCE demonstrated LPS dose-dependent changes in oxidation current which enabled selective quantitative detection. This method exhibited a detection limit as low as 8.4x10(-4) ng mL(-1) with a wide linear range of 0.01-2.0 ng mL(-1). For continuous on-chip monitoring of LPS, a customized microfluidic electrochemical device approach was established which endorses the practical utility of the developed sensor. Further, the integrated microfluidic electrochemical device showed excellent recovery upon spiking LPS in lemon juice and tap water samples. Overall, the fabricated sensor holds great promise for point-of-care screening of trace LPS contamination, especially for food safety.